The present invention relates to x-ray imaging of objects for medical, industrial and scientific applications; for example, it relates to the medical imaging of the human body, and the x-ray inspection of objects to determine content. New methods are presented for the fabrication of thin lenses by replicating high quality optical surfaces onto unit lenses for compound refractive lens systems for the focusing, collection, collimation, and general manipulation of x-rays. These lens systems will have medical, industrial and scientific applications.
a. Compound Refractive Lenses for X-Rays
Ordinarily a single lens would not appreciably affect x-rays. However, if many lenses of the same focal length, ƒ, are stacked one after another, then the focal length can be reduced by the reciprocal of the number of lenses, ƒ/N. This well-known effect is often utilized for optics in the visible spectrum as described for example in the text book by E. Hecht and A. Zajac, Optics, (Addison-Wesley, 1974), Chap. 5, pp. 99-166. For x-rays, the index of refraction is generally slightly less than 1; therefore, concave lenses will focus x-rays. If a single bi-concave lens has a focal distance of                               f          =                      R                          2              ⁢              δ                                      ,                            (        1        )            
where R is the radius of curvature on-axis of the lens or the radius of the sphere, if the lens is spherical, and xcex4 is a small number in the order of 10xe2x88x927 to 10xe2x88x925 for most materials at x-ray wavelengths. xcex4 is the decrement of the complex index of refraction of the lens material expressed by
n=1xe2x88x92xcex4xe2x88x92ixcex2,xe2x80x83xe2x80x83(2) 
A compound lens made up of N lenses has a focal length equal to                     f        =                              R                          2              ⁢                              xe2x80x83                            ⁢              N              ⁢                              xe2x80x83                            ⁢              δ                                .                                    (        3        )            
The focal length is reduced by 1/N. Thus, if a single lens had a focal length of 100 meters at a given radiation wavelength, a compound refractive lens of 100 unit lenses will give a 1 meter focal length. A prior art compound refractive lens is illustrated in FIG. 1, where a number of unit lenses are stacked in a series. The x-rays pass through each of the unit lenses 10. Every lens focuses the x-ray only slightly, but the total series of lenses will result in appreciable x-ray focusing. Focal lengths of less than 20 cm have been achieved for 8 keV x-rays.
In the prior art very low Z materials were suggested to be best for unit lenses. Beryllium was suggested by Yang (B. X. Yang xe2x80x9cFresnel and refractive lenses for X-raysxe2x80x9d, Nuclear Instruments and Methods in Physical Research A328 pp. 578-587 (1993)) to be the best material for making lenses. Yang""s paper states that the best material possesses a large xcex4/xcex2, where xcex2 and xcex4 are the factors in the complex dielectric constant as given by Eqn. 2. This is roughly a measure of how much the material can bend x-rays relative to the amount of absorption. Both lithium and beryllium give large xcex4/xcex2, and are deemed the best lens materials. Unfortunately, these metals have properties that make them both difficult to fabricate and to use. Lithium is very hygroscopic, chemically reactive, and easily bonds to other metals (such as debossing or compression molding tools made of brass or steel). Beryllium is expensive and extremely difficult to machine, and becomes highly toxic if airborne during the machining process.
In U.S. Pat. No. 6,269,145 B1, May 1998, by M. A. Piestrup, R. H. Pantell, J. T. Cremer and H. R. Beguiristain, entitled xe2x80x9cCompound Refractive Lens for X-rays,xe2x80x9d lenses were made by compressing steel balls or metal compression molding pins on both sides of a thin plastic film and thin metallic sheets. (That patent is incorporated herein by reference.) The two steel balls or molding pins are arranged diametrically opposite one another and are pressed against the surface of the film forming a biconcave lens. The quality of the surface (or surface roughness) of the lens matches that of the surface of the ball or molding pin. Thus, by having high quality stainless steel balls or pins, one can achieve a high quality lens surface, which is critical in obtaining high quality x-ray focusing and imaging. Parabolic and other surfaces, such as Fresnel lenses, can be machined into brass and steel pins to make debossing or compression molding tools for making unit lenses.
In U.S. Pat. No. 6,269,145 B1, May 1998, by M. A. Piestrup et al, methods of stacking, aligning and containing the unit lenses are described.
In the literature (E. M. Dufresne, D. A. Arms, R. Clarke, N. R. Pereira, S. B. Dierker, D. Foster, xe2x80x9cLithium Metal for X-ray Refractive Opticsxe2x80x9d, Appl. Phys. Lett., 79, 4085 (2001).), lithium lenses using a saw-tooth pattern have been demonstrated to focus x-rays. Separate individual unit lenses (xe2x80x9ccoin lensesxe2x80x9d) of lithium have not been fabricated because of difficulty of fabrication, environmental degradation of the lithium, and lens surface quality.
In accordance with preferred embodiments of the invention, a compound refractive lens for x-rays is provided which is made up of a plurality of individual unit lenses comprising a total of N in number, said unit lenses hereinafter designated individually with numbers i=1 through N, said unit lenses substantially aligned along an axis, said i-th lens having a displacement ti orthogonal to said axis, with said axis located such that             ∑              i        =        1            N        ⁢          t      i        =  0.
In addition each of said unit lenses is made up of a lens material selected from the group consisting of lithium, carbon, and polyimide, and having a refractive index decrement xcex4 less than 1 at a wavelength xcex less than 50 Angstroms.
A nice feature of the present invention is that if provides for a stack of individual thin lenses (xe2x80x9ccoinxe2x80x9d lenses) that have high quality surfaces. The invention permits the inexpensive fabrication of unit lenses for compound refractive lenses with appropriately shaped, high-quality, optical surfaces, such as paraboloidal, spherical or more complex required shapes such as Fresnel lenses. These unit lenses can be made of soft materials such as carbon, plastics, soft metals such as lithium, aluminum, and even harder metals such as beryllium. The invention permits the inexpensive fabrication of lithium unit lenses without the lithium sticking to the debossing or compression molding (lens shaping) tool. The invention permits the fabrication of optical quality unit lenses with optical surfaces that can minimize compound refractive lens aberrations and that may also improve image quality of the lenses such as paraboloidal, spherical or more complex optimized surfaces.
The invention also provides for an adequate housing for the unit lenses such that they are held in coaxial alignment, and are in a chemically inert environment, such as an inert gas or vacuum or dry-air environment, whereby the lithium lenses do not experience chemical change due to the presence of moisture or any other chemically active atoms or molecules.